Referring to the blowing technology of glass container with a long history, the plastic mould in viscoelastic extrusion or injection molding is placed in the blowing mould, and the mould is expanded by compressed air to make it close to the cavity wall. After cooling and finalizing, the mould is demoulded to obtain the hollow plastic product with the same shape as the cavity. This is the hollow blow molding process.
At present, blow molding has developed into the first production mode of plastic bottle making. The advantages of blow molding are that the cost of mould and die is low, the material of mould is rich, the variety of products is rich, large and complex hollow products can be produced, and the production efficiency and automation degree are high. The disadvantages include the limitation of product shape, the difficulty of wall thickness control, and the low dimensional accuracy.
Common blow molding processes include extrusion blow molding, injection blow molding and stretch blow molding. Extrusion blow molding: extrusion device extrudes molten materials, and the extrusion mold embryo is placed in the blow molding mold for inflation, and the product is demoulded and taken out after cooling and finalizing; injection blow molding: the bottom mold embryo is injected first, and then transferred to the blow molding mold for inflation, and the product is demoulded and taken out after cooling and finalizing; stretch blow molding is the production mold embryo by extrusion or injection and placed in the blow molding mold Use the stretching rod to stretch axially, blow air to stretch transversely, then cool and finalize, finally demould and take out the product.
Extrusion blow molding
Extrusion blow molding is the largest and earlier blow molding method. It is suitable for blow molding of PE, PP, PVC, thermoplastic engineering plastics, thermoplastic elastomers and other polymers as well as various blends. It is mainly used to produce packaging containers, storage tanks and other industrial parts.
In the extrusion blow molding process, firstly, the material is extruded from the die head after melting and plasticizing in the extruder; when the die reaches the predetermined length, a section of the die is cut off and placed in the blow molding die while it is hot, and the die is closed and clamped; then, the precise shape of the blow molding cavity is blown by the compressed air blown into the die through the blowing nozzle, and then the product can be taken out after the die is opened.
In the extrusion blow molding process, the production cost of the container is relatively low, and the capacity of multi-layer, thin-walled and complex containers can be produced in the range of 1ml-3000l. But in the production process, there will be flash, unstable screw tooth size, poor sealing, poor automation, long product cycle and low production efficiency.
However, the extrusion blow molding process has obvious advantages in producing thin-walled products, tubular or bottle bodies with complex shapes by virtue of the two technologies of billet wall thickness controller and core free design.
The wall thickness controller can adjust the wall thickness distribution of the blank according to the blow out ratio of each part of the blank, and blow out the products with uniform thickness.
Taking the automobile industry accessories as an example, the door panel, roof and the common seamless door handle of high-end cars produced by extrusion blow molding process are light, thin and strong, reducing fuel consumption and manufacturing cost. Therefore, more and more auto parts adopt extrusion blow molding technology, such as the production of fuel tank and various pipe fittings.
Coreless design is very popular in the field of household chemicals. Take the common large volume daily chemical bottle as an example. Due to its large size and heavy quality, customers will give priority to products with handles. The coreless design does not need to consider the problem of demoulding and clamping. It can produce containers with handle structure, even folding containers with complex structure.
Injection blow molding is the combination of injection molding and blow molding: first, the injection agent injects the melt into the injection mold with blowing core tube to form the mold with bottom, and transfers the mold to the mold with blowing station through mechanical device for blowing; then, the blow molding mold introduces the compressed air from the core hole into the mold, and the mold is close to the blow molding cavity under the effect of air pressure After cooling, the product is transferred to the demoulding station; finally, the product is ejected from the mandrel or directly sent to the packaging station for production in the next cycle.
The injection blow molding process has a high degree of automation and production efficiency; the products have no splicing and wiring, high bottom strength, uniform wall thickness and high accuracy of mouth size; there is no corner scrap or flash, and the seam line is thin; it is suitable for processing large quantities of small refined containers and wide mouth containers. Its disadvantage is that it is unable to process products with too complex shape; the container capacity is limited to 5ml-500ml.
In addition, due to the use of hydraulic drive linkage structure, the positioning of the machine is more accurate, the maintenance cost is lower, and more rapid and stable. The demoulding station is equipped with conveyor belt to realize automatic demoulding, automatic counting and automatic packaging, which can meet the strict requirements in the field of medicine and health.
